Broad band amplifier



Sept. 26, 1939.

M. CAWEIN BROAD BAND AMPLIFIER Filed May 11, 1937 INVENTOR MA SON CAWE ATTORNEY Patented Sept. 26, 1939 UNITED STATES arises;

nnosn mum min-ma Madison Gawein, Manhasset, N. 1., assignor to Hazeltine Corporation, a corporation of Delaware Application May 11, 1937, Serial No. 141,916

5Claims.

quency and becomes appreciable at the higher.

frequencies, materially reducing the gain of the tube. This effect is accentuated where, in one of the stages, 'two or more vacuum tubes are operated in parallel, as in a combining amplifier, the internal anode-cathode capacitances of the tubes also being in parallel. At the same time, in such an amplifier it is particularly important that the load impedances of the several tubes be maintained at their maximum values to prevent cross modulation and distortion.

In addition, conventional multistage vacuumtube amplifiers reverse the phase of the input voltage in each stage, thus requiring an even number of stages if a resultant phasereversal is to be avoided. This feature is often objectionable where an odd number of stages is adequate.

It is an object of the invention, therefore, to provide a multistage broad-band vacuum-tube amplifier which is effective uniformly to amplify all frequencies of the band.

It is a further object of the invention to provide a multistage vacuum-tube amplifier in which a singleresultant phase reyersal is obtained for any number of amplifier stages.

In accordance with this invention, a multistage broad-band amplifier comprises two or more impedance-coupled vacuum tubes, the impedance means included in the output circuit of each stage being coupled to the cathode of the vacuum tube or tubes of the succeeding stage so that, for direct current, the vacuum tubes are effectively in series. For alternating currents they are in cascade, but the input voltage to each stage after the first is reversed in phase with respect to conventional multistage amplifiers so that there is a single resultant phase reversal, regardless of the number of stages. In a preferred form of the invention, illustrated and described hereinafter, it is applied to a television combining amplifier for combining and amplifying a broad band of video frequencies, line-frequency impulses, and frame-frequency impulses prior to modulation of the carrier wave.

The single figure of the drawing is a circuit diagram of an embodiment of the invention as applied to a combining amplifier suitable for use in a television transmitter.

Referring now to the drawing, there is shown a multistage broad-band amplifier for combining and amplifying signals received from a videofrequency input circuit H, which may be energized from a cathode-ray signal-generating tube either with or without intermediate amplification, a line-frequency generator Ii, and a frame-frequency generator l2, and for delivering the combined output to a load device l3, which may be a modulator. The video-frequency input circuit III is coupled to the input circuit of a vacuum tube M, while the line-frequency generators i l and I2 are coupled to the input circuits of vacuum tubes l5 and [6, respectively, the tubes I, I5, and I6 comprising the first stage of the amplifier. The anodes of the tubes ll, l5, and it are connected in parallel, while their cathodes are grounded through conventional biasing circuits I9, 20, and 2|, respectively. The sum of the anode-cathode capacitances of tubes ll, l5, and I6 is represented by the dotted-line condenser 26. Power is supplied to the tubes l4, l5, and it from a source indicated as +B, through load resistors 22 and 23 in series, resistor 22 being by-passed by condenser 21.

The second amplifier stage comprises vacuum tube IT having its grid grounded for alternating potentials through condenser 29 and its cathode connected to the common anode terminal of the tubes l4, l5, and it of the first amplifier stage. The output circuit of the first amplifier stage is untunable and substantially all of its impedance is included in a common portion of the anode-cathode and grid-cathode circuits of the second amplifier stage. The output circuit of tube I1 is coupled to the input terminals of load device l3. Power is supplied to the tube l'l' from the source indicated at +3, through a load resistor 24. The control grid of tube H is biased by means of a voltage divider comprise ing resistors 25 and 28 and connected across the source +B, as shown in the drawing.

In considering the operation of the system described, .it will be assumed that suitable signal inputs are being supplied from the sources III, II, and I2 and that suitable operating potentials are impressed upon the tube electrodes. As a result, the voltage appearing across resistor 23 in the common output circuit of tubes ll, l5, and It contains three components, video-frequency voltages and line-frequency and framefrequency impulses, but without substantial cross modulation or distortion. The periodic voltage appearing across resistor 23 is impressed between the cathode of tube ii and ground through condenser 21, while the grid of tube [1 is grounded for periodic voltage through condenser 29. As a result, the phase of the input voltage to the tube I1 is reversed with respect to that in a conventional impedance-coupled amplifier and the output voltage supplied to the load I3 is substantially in phase with that developed in the common output circuit of tubes l4, l5, and 16.

As stated above, the combined interelectrode capacitance, represented by the condenser 26, constitutes a shunt admittance which rises with frequency so that, at the higher frequencies of the band, the gain of the first stage is materially impaired, resulting in a nonuniform amplification throughout the broad band of frequencies to be passed by the system. In the second amplifier stage, resistor 23 is common to the gridcathode circuit and the anode-cathode circuit of tube l1 and, accordingly, produces degeneration in the second amplifier stage. However, the combined interelectrode capacitance 26 of the first stage effectively by-passes the resistor 23 to an extent which increases with frequency so that the second amplifier stage is degenerative primarily at low frequencies. Therefore, the nonuniform gain characteristic of the first amplifier stage, which discriminates in favor of the lower frequencies, is partially compensated by the nonuniform gain characteristic of the second amplifier stage, which discriminates in favor of the higher frequencies. It has been found that, by properly selecting the circuit constants, the two gain characteristics may be made substantially complementary so that the amplifier transmits a broad band of frequencies with a relatively uniform over-all gain.

While the invention is of general application, the following circuit constants are given as illustrating a specific embodiment of the invention which has been found to produce the desired over-all gain characteristic:

Range of video-frequency input cycles 10 to 1,600,000 Frequency of line-frequency generator 11 cycles..- 13,290 Frequency of frame-frequency generator 12 cycles 60 Tubes 14, 15, 16, and 17 type 6C6 Value of resistor 23 ohms 5,000 Value of resistor 24 do 10,000 Value of resistor 22 do 25,000 Value of resistor 25 megohm 1 Value of resistor 28 dc 1 Value of condenser 27 f 8.0 Value of condenser 29 ILf 0.1

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A multistage broad-band vacuum-tube amplifying system comprising a first vacuum-tube amplifying stage having an untunable output circuit with appreciable shunt capacitance, whereby the higher frequencies of the band are discriminated against, a second vacuum-tube amplifying stage having anode-cathode and gridcathode circuits including a common portion,

and shunt impedance means in the output circuit of said first stage and coupled in the common cathode circuit of said second stage, said shunt capacitance causing said second stage to be degenerative primarily at the lower frequencies of the band to discriminate thereagainst.

2. A multistage broad-band vacuum-tube amplifying system comprising a first vacuum-tube amplifying stage 'having an untunable output circuit with appreciable shunt capacitance, a second amplifying stage in cascade with said first stage and comprising anode-cathode and gridcathode circuits including a common portion, and an appreciable impedance variable with frequency over said band in the output circuit of said first stage and coupled in the common cathode circuit of said second stage, whereby said amplifier stages have complementary response characteristics.

3. In a multistage broad-band vacuum-tube amplifier, a first stage comprising a plurality of vacuum tubes having output electrodes in parallel in an untunable output circuit, whereby the anode-cathode capacitances of said tubes are also in parallel and constitute an appreciable shunt capacitance and the higher frequencies of the band are discriminated against, a second vacuum-tube amplifying stage having anodecathode and grid-cathode circuits including a common portion, and shunt impedance means in the output circuit of said first stage and coupled to the common cathode circuit of said second stage, said shunt capacitance causing said second stage to be degenerative primarily at the lower frequencies of the band to discriminate thereagainst.

4. A multistage broad-band vacuum-tube amplifying system comprising a first vacuum tube amplifying stage having an output circuit with appreciable shunt capacitance, whereby the higher frequencies of the band are discriminated against, a second vacuum-tube amplifying stage having anode-cathode and grid-cathode circuits including a common portion, said stages being connected effectively in series for direct current and in cascade for alternating currents, and impedance coupling means including substantially all of the impedance of said output circuit of said first stage including said capacitance coupled in the common cathode circuit of said second stage to cause said second stage to be degenerative primarily at the lower frequencies of the band to discriminate thereagainst.

5. A multistage broad-band vacuum-tube amplifying system comprising a first vacuum-tube amplifying stage having an inherent shunt capacitance in the output circuit thereof, whereby the higher frequencies of the band are discriminated against, and a second vacuum-tube amplifying stage having anode-cathode and gridcathode circuits including a common resistor, said stages being connected effectively in series for direct current and in cascade for alternating currents, said capacitance being effectively in shunt with said resistor, said resistor and said capacitance comprising substantially all of the impedance of said output circuit, whereby said second stage is degenerative primarily at the lower frequencies of the band and discriminates thereagainst.

MADISON CAWEIN. 

